Eyepieces are known which include an optical arrangement known as a pancake window. These known eyepieces are used to view flat panel displays but they only have a limited field of view.
In any eyepiece, there is a zone in space where the eye pupil can be placed in order to view the image. The zone is defined by the distance between the last face of the last optical element, called the eye-relief, and an area at this distance where the image can be seen. If the eye pupil is moved laterally outside this area, some optical systems exhibit an exit pupil where a sudden transition from image to darkness would be observed. Other systems have a less clear transition where, as the eye is moved outside the design viewing area, the image is observed to lose brightness and/or resolution.
In applications that require a wide field of view, a small viewing area is undesirable because the eye may swivel in order to vary the point of regard within the view. In addition, if the optical system is head mounted as opposed to hand held, it is not readily possible to compensate for this eye movement by moving the eyepiece relative to the eye. The situation is further complicated by the fact that most methods of mounting the display on the head permit some relative movement. With conventional eyepiece design, the need for a wide field of view, reasonable eye-relief, a large viewing area, and a compact lightweight eyepiece are incompatible.
When an eyepiece is used in a head mounted display, the head mounted display forms a means for interacting with a three dimensional virtual environment. In order to form the virtual environment, head tracking devices are used to monitor the position and orientation of the user's head, information from which is fed back to an image generator such that the image displayed corresponds to the position and the orientation within a computer model. Head mounted displays with a large field of view give the impression of being in the image, as opposed to watching on a screen. Optical solutions for the eyepiece tend to trade the desired field of view for viewing area size. Moreover, the eye relief can only be scaled up by increasing the size and therefore the weight of the eyepiece. Still further, refractive eyepieces are complicated, requiring many large and heavy elements, positioned in some cases well in front of the eye, which mitigates against an ideal weight distribution.
The desired total field of view for a head mounted display is 120°×67°. Known head mounted displays that have been designed to have a large field of view have been both very heavy and very expensive. The head mounted displays are usually unbalanced, with most of the weight being to the front of the user's head. The excessive weight of the known head mounted displays has made them uncomfortable to use and inefficient. This is because, in order to work effectively, the eyepiece must be in a fixed location relative to the user. The excessive weight of the head mounted display means that inertia is substantial and tends to cause the head mounted display to lag when the user's head makes an angular acceleration, and to overshoot as the user's head decelerates.